Heat exchangers constituted from a plurality of multi-hole flat tubes having formed in the interior thereof a plurality of refrigerant flow channels, and a plurality of flat tubes through the interior of which flows another heating medium, stacked in alternating fashion, exist in the prior art. As disclosed, e.g., in Japanese Laid-open Patent Application 2007-17133, such heat exchangers are constituted such that the ends of the respective multi-hole flat tubes connect to a header which extends in a direction intersecting a lengthwise direction of the multi-hole flat tubes, the refrigerant flow channels of the respective multi-hole flat tubes communicating via the internal space of the header.